The present invention relates to error correction circuits, and more particularly to an avalanche photodiode (APD) non-linearity cancellation circuit for cancelling a portion of the output current tail from the APD using the fact that the current tail is only mildly dependent upon the gain of the APD.
When an optical signal, which is received by an APD, changes amplitude abruptly from a high to a low level, the output current of the APD exhibits a non-linear recovery, or tail, from the high level signal that swamps any low level signal that also may be present. This phenomenon limits, for example, the spatial resolution of optical time domain reflectometers.
In optical time domain reflectometers there have been different masking techniques used to attempt to limit this tail. For example in U.S. Pat. No. 4,769,534, issued Sept. 6, 1988 to Donald L. Brand entitled "Optical Detector with Storage Effects Reduction", an optical switch is interposed between the received optical signal and the optical detector to mask the high level signals from being passed to the optical detector during non-sampling intervals. The optical switch is controlled so that the switch closes only when the received optical signal is to be samples.
Another method using an optical switch is disclosed in co-pending U.S. patent application No. 07/379,852, filed by Kevin B. McDonald on July 14, 1989 entitled "Automatic Mask Trigger for an Optical Time Domain Reflectometer", where a portion of the electrical output signal is compared with a portion of the optical input signal to generate a switch control signal to pass the optical input signal, after a delay interval, to the optical detector only in the absence of high level signals.
Manual methods also have been employed that require an operator to observe an initial acquisition of data, and then via a cursor or other means to signal the optical switch to turn off on subsequent data acquisitions at high level times.
What is desired is a technique for automatically correcting for the non-linearity of the output tail of an APD in response to the transition from a high signal to a low signal in order to increase the spatial resolution of an optical time domain reflectometer.